Electrochemical cells provide electrical energy that powers a host of electronic devices such as external and implantable medical devices. Among these many medical devices powered by electrochemical cells are external medical drills and implantable cardiac defibrillators. Such medical devices generally require the delivery of a significant amount of current in a relatively short duration of time. Thus, these devices typically require the use of electrochemical cells that comprise an increased delivery capacity and an increased rate of charge delivery. As defined herein, “delivery capacity” is the maximum amount of electrical current that can be drawn from a cell under a specific set of conditions. The terms, “rate of charge delivery” and “rate capability” are defined herein as the maximum continuous or pulsed output current a battery can provide per unit of time. Thus, an increased rate of charge delivery occurs when a cell discharges an increased amount of current per unit of time in comparison to a similarly built cell, but of a different anode and/or cathode chemistry.
Cathode chemistries such as carbon monofluoride (CFx) have been developed to provide increased discharge capacities that meet the power demands of external and implantable medical devices. CFx cathode material is generally known to have a discharge capacity of about 875 mAh/g, which is well suited for powering implantable medical devices over long periods of time. However, electrochemical cells constructed with cathodes comprised of carbon monofluoride are generally considered to exhibit a relatively “low” rate capability. For example, electrochemical cells constructed with lithium anodes and CFx cathodes typically exhibit rate capabilities from about 0.5 mA/cm2 to about 3 mA/cm2. As such, electrochemical cells constructed with Li/CFx couples are generally well suited for powering electrical devices, like an implantable cardiac pacemaker that are powered over long periods of time at a relatively low rate capability.
In contrast, electrochemical cells constructed with lithium anodes and cathodes comprising silver vanadium oxide (SVO) are generally considered to exhibit a relatively “high” rate capability. Lithium cells constructed with SVO cathodes, in contrast to CFx cathodes, generally exhibit rate capabilities that range from about 25 mA/cm2 to about 35 mA/cm2. As such, lithium electrochemical cells constructed with cathodes comprised of SVO are generally well suited to power devices that require an increased rate capability, such as an implantable cardiac defibrillator. However, lithium cells constructed with cathodes comprising SVO typically have a lower discharge capacity as compared to those having cathodes comprising CFx. Silver vanadium oxide cathode material is generally known to have a discharge capacity of about 315 mAh/g, which is significantly less than the discharge capacity of 875 mAh/g for CFx as previously discussed. Therefore, what is desired is a cathode material and electrochemical cell thereof that comprises a “high” discharge capacity in addition to an increased rate capability. Such an electrochemical cell would be well suited for powering additional electronic devices that require an increased charge capacity with an increased discharge rate.
The applicants, therefore, have developed a new iron cobalt disulfide cathode material formulation and cathode thereof that provides a lithium electrochemical cell with a discharge capacity of between about 700 mAh/g to about 850 mAh/g and an increased rate capability of between about 15 mA/cm2 to about 20 mA/cm2. Thus, a cathode composed of the iron cobalt disulfide material of the present invention when constructed within an electrochemical cell having a lithium anode is well suited for powering a variety of electrical devices that require a “high” discharge capacity with an increased rate capability.
The use of iron disulfide as a cathode material is generally known. In particular, Awano et al. in “Li/Fe1-xCoxS2 System Thermal Battery Performance” Power Sources Symposium 1992, p. 219-222 disclose an iron cobalt disulfide cathode material having a general formula of Fe1-xCoxS2, wherein x≤0.4. The Awano et al. material is fabricated by mixing dry iron powder, metal cobalt powder and sulfur together. This is followed by subjecting the powder mixture to a temperature of between 350° C. to 550° C. in an argon gas environment. This method is similar to processes commonly used in the industry where metal powders are combined and subsequently heated under a gaseous flow.
In contrast, the iron cobalt disulfide cathode material of the present invention is fabricated using a hydrothermal process in which iron sulfate, cobalt sulfate, sulfur are mixed in an aqueous mixture at a temperature of about a range from about 100° C. to 300° C., preferably about 200° C. As a true hydrothermal process, the synthetic method of the present invention uses metal salts that are dissolved and reacted in a liquid phase rendering a unique chemical structure. The reaction can therefore take place at much lower temperatures and without the need of gas flow, as is required by the prior art, for example, the Awano et al. process. The present invention process has the advantage of producing a very homogeneous product without the cost and complexity of the exemplary Awano et al. commercial technology.
In addition, the Awano et al. cathode material comprises a chemical structure that is different than the cathode material of the present invention. Specifically, the iron cobalt disulfide material of the present invention comprises an increased amount of cobalt as compared to Awano et al. Furthermore, the lattice structure of the iron cobalt disulfide material of the present invention decreases in size with an increasing amount of cobalt. In contrast, the Awano et al. iron cobalt disulfide material comprises a lattice structure that increases in size with increasing amounts of cobalt.
These chemical and structural differences between the iron cobalt disulfide cathode materials of Awano et al. and that of the present invention manifest themselves in electrical performance differences when constructed within a lithium electrochemical cell. For example, lithium cells constructed with the Awano et al. cathode material exhibit an increased background voltage as compared to lithium cells constructed with the cathode material of the present invention. Thus, as will be discussed in more detail, the iron cobalt disulfide cathode material of the present invention comprises a unique chemical structure that provides a lithium electrochemical cell with electrical properties that are well suited to power a variety of electrical devices that require an increased discharge capacity with increased rate capability.